The present invention relates generally to electronic patient monitors and, in particular, to patient monitors suitable for use in the severe electromagnetic environment of a magnetic resonance imaging machine.
Magnetic resonance imaging (MRI) allows images to be created of soft tissue from faint electrical resonance (NMR) signals emitted by nuclei of the tissue. The resonance signals are generated when the tissue is subjected to a strong magnetic field and excited by a radiofrequency pulse.
The quality of the MRI image is in part dependent on the quality of the magnetic field, which must be strong and extremely homogenous. Ferromagnetic materials are normally excluded from the MRI environment to prevent unwanted magnetic forces on these materials and distortion of the homogenous field by these materials.
A patient undergoing an MRI “scan” may be received into a relatively narrow bore, or cavity, in the MRI magnet. During this time, the patient may be remotely monitored to determine, for example, heartbeat, respiration, temperature, and blood oxygen. A typical remote monitoring system provides “in-bore” sensors on the patient connected by electrical or optical cables to a monitoring unit outside of the bore.
Connecting an in-bore sensor to a monitoring unit may be done with long runs of electrical or optical cables. Such cables can be a problem because they are cumbersome and can interfere with access to the patient and free movement of personnel about the magnet itself.
One solution to these problems of cabling is described in co-pending U.S. patent application Ser. Nos. 11/080,958, filed Mar. 15, 2005, and 11/080,743, filed Mar. 15, 2005, assigned to the assignee of the present invention and hereby incorporated by reference. This patent application describes a wireless patient monitor that may be positioned near the patient to provide real-time monitoring of patient physiological signals during the MRI examination.
The electrical environment of the MRI system produces a considerable amount of electrical noise that may interfere with the collection of physiological signals by in-bore sensors. The noise may include that generated by radio frequency pulses used to stimulate the protons of the patient's tissue into precession and noise induced by rapidly switched gradient magnetic fields. The character of the electrical noise produced by the MRI machine may change depending on the machine and on the particular scan protocols.
In order to reduce the effect of the electrical noise on the acquired physiological signals obtained by in-bore sensors, it is known to provide different types of signal filters, appropriate for different imaging situations, at the monitoring unit outside the bore. An operator observing the physiological signals may select among the different filters to choose a filter that provides a physiological signal with the lowest noise.
When wireless in-bore sensors are used, the transmitted signal must generally be pre-filtered before transmission to limit the bandwidth of the transmitted signal. This pre-filtering is not intended to separate all noise from the physiological signal and generally will not provide the entire optimum filter for this purpose. Nevertheless, to the extent that the pre-filter necessarily modifies the transmitted signal, it limits the ability of the operator to fully assess the amount of noise on the physiological signal and/or to fully control the filtration of the physiological signal.